1. Field of the Invention
The present invention relates to a correlation detecting apparatus and a method for detecting a specific code contained in a received signal, and particularly relates to a code detecting apparatus and a method which are preferably applied to a case where a timing connects a plurality of terminals in a wireless fashion.
2. Description of the Prior Art
As the function of a computer becomes higher, it is now popularized to construct a LAN (Local Area Network) by connecting a plurality of computers in order to share files or data, or transfer electronic mails or data. The conventional LAN employs an optical fiber or a coaxial cable, otherwise a twist pair cable to connect each computer by a wire.
However, in such a wired LAN, connecting works are required, thus making it difficult to readily construct the LAN and besides, in the wired LAN the cables become complicated. Therefore, a wireless LAN is now attracting attention as a system which releases a user from wiring of the conventional wired LAN.
As the wireless LAN, such a version that data communication is performed on a CDMA (Code Division Multiple Access) system using a spread spectrum has heretofore been proposed. In the CDMA system, transmitting data is multiplied by a PN (Psuedo Noise Code) code, thereby causing the spectrum of transmitting data to be spread. Transmitted data with such a spread spectrum is demodulated by multiplying the similar PN code similar to the transmitting side. This CDMA system is characterized by high privacy property and excellent interfere-resistance.
Recently, information is increasingly turned into multimedia, so that a large amount of data such as video data and audio data is now often handled. For this reason, it has also been demanded for the wireless LAN to make a speed of transfer rate higher so that a large amount of data such as video data and audio data can be transmitted. Unfortunately, in the spectrum spread modulation, if data transfer is performed at a high rate, e.g. about 30 Mbps, a band width of 300 MHz or more will then be required. Such a wide band width cannot be ensured with the present frequency allocation and so it is difficult to secure such a wide band width for making communication.
Moreover, with the spectrum spread method, in order to demodulate, a synchronism capturing time is required for matching the phase of transmitted data code with the phase of a code generated to demodulate in a receiver. For this reason, in the spectrum spread method, a synchronizing bit sequence is inserted in each packet in order to capture the synchronism at a high speed. Due to such a synchronizing bit sequence, a problem in which bits other than effective data increase will arise.
Therefore, the applicant of this application has proposed that data is transmitted on an OFDM (Orthogonal Frequency Division Multiplexing) system, data communication being performed on a TDMA (Time Division Multiple Access) system making one frame as a unit, an M sequence being transmitted at the head of one frame, a transmitting/receiving time being determined with reference to this M sequence and the transmitting/receiving time of each wireless communication terminal being indicated by control information from a wireless communication control terminal. According to the OFDM system, the transfer rate can be raised and the correct demodulation can be achieved even if any jitter arises. Furthermore, because the transmitting/receiving timing is set up with reference to the M sequence at the head of one frame, data can be reproduced during reception by demodulating only a required symbol within a frame using that time information.
Where the M sequence is received and the timing is set in this manner, it is necessary to detect the M sequence from a received signal. For a circuit to detect such the M sequence, it is conceived to employ a correlation detector circuit using a matched filter.
FIG. 1 shows an example of the correlation detector circuit using the matched filter as described above. Referring to FIG. 1, a received signal from an input terminal 151 is supplied to a matched filter 152. The matched filter 152 is a sort of an FIR filter and, as is shown in FIG. 2, is comprised of delay circuits 161-1, 161-2, 161-3, . . . , multiplier circuits 162-1, 162-2, 162-3, . . . and an adder circuit 163. Coefficients of the multiplier circuits 162-1, 162-2, 162-3, . . . are set to 1 or (xe2x88x921) depending on a code to be detected. If a strong correlation exists between codes which are set as the coefficients of the multiplier circuits 162-1, 162-2, 162-3, . . . and the received code, an output level of the adder circuit 163 will rise.
Thus, the matched filter 152 detects the correlation between the received code and the code set for the filter. The output of the matched filter 152 is supplied to a comparator circuit 153. The comparator circuit 153 compares the output of the matched filter 152 with a predetermined threshold value TH.
If the code of M sequence is received, the output of matched filter 152 rises and the output of the matched filter 152 exceeds the threshold value TH. If the output of matched filter 152 exceeds the threshold value TH, a detecting output occurs at an output terminal 154.
In such a system, however, signals from a plurality of wireless communication terminals are time division multiplexed within the frame for transmission and so different levels of signals are received from the respective wireless communication terminals and a wireless communication control terminal. Therefore, even if the matched filter 152 receives the M sequence and yields the received output of the M sequence, this signal output may be hidden by other signal components. This will raise a problem that the M sequence signal cannot be detected.
Specifically, the received signal is subjected to an AGC (Automatic Gain Control) and controlled so that the received signal level may be constant. Thus, when any signal with an excessive amplitude is input immediately before the M sequence signal, then the AGC will act to reduce its gain. If the M sequence signal is received in a condition that the gain is reduced, the detected level of correlation signal of M sequence will be lowered and buried in a strong received signal, thus making it impossible to be detected. Moreover, even though only a portion near an area where the M sequence is received may be extracted, the detected level of correlation signal of M sequence will turn below the threshold value, thereby making it impossible to detect the M sequence signal.
In this way, where the plurality of signals with different signal levels are transmitted on the time division fashion, it is difficult to ensure the detection of M sequence signal with such a construction as compares the output of matched filter with a predetermined threshold value.
Therefore, an object of the present invention is to provide a correlation detecting apparatus which is capable of ensuring the detection of a code sequence even if a plurality of signals with different amplitudes are time division multiplexed.
The present invention provides a correlation detecting apparatus comprising a matched filter means for detecting a predetermined code from a received signal, an average amplitude detecting means for detecting an average amplitude of the received signal, a normalizer means for normalizing an output of the matched filter means by an output of the average amplitude detecting means and a comparator means for comparing the normalized output of the matched filter means by the normalizer means with a threshold value to output a correlation detecting signal and its method.
Further, the present invention provides a communication apparatus comprising an input means inputted with a received signal, a synchronizing signal detecting means for detecting a synchronizing signal contained in the received signal from the received signal, and a demodulating means for demodulating and outputting the received signal based on the synchronizing signal, in which the synchronizing signal detecting means is provided with a detecting means for detecting a predetermined signal from the received signal, an average amplitude detecting means for detecting an average amplitude of the received signal, a normalizing means for normalizing an output of the detecting means based on an output of the average amplitude detecting means, and a comparing means for comparing an output of the detecting means normalized by the normalizing means with a threshold value to thereby output a correlation detecting signal, and its communication method.
Also, the present invention provides a communication apparatus comprising an input means inputted with a received signal, a synchronizing signal detecting means for detecting a synchronizing signal contained in the received signal from the received signal, and a transmitting means for transmitting data based on a detecting timing of the synchronizing signal, in which the synchronizing signal detecting means is provided with a detecting means for detecting a predetermined signal from the received signal, an average amplitude detecting means for detecting an average amplitude of the received signal, a normalizing means for normalizing an output of the detecting means based on an output of the average amplitude detecting means, and a comparing means for comparing an output of the detecting means normalized by the normalizing means with a threshold value to thereby output a correlation detecting signal, and its communication method.
The correlation detecting apparatus compares the divided and normalized value of the output of matched filter by the average amplitude of the received signal with the threshold value in order to detect the code of M sequence from the received signal. Because the output of matched filter is directly proportional to an input signal level, the output increases not only when the M sequence is received but also when the received signal level is large. When the received signal level is large, the output level of matched filter increases. On this occasion, the average amplitude of received signal also increases. For this reason, the divided value of the output level of matched filter by the output level of average amplitude detector circuit makes almost no change. In contrast, the output level of matched filter when the M sequence is received remains nearly constant irrespective of the input signal level. Therefore, by comparing the divided and normalized value of the output of matched filter by the average amplitude of the received signal with the threshold value to detect the M sequence code from the received signal, it is enabled to securely detect the M sequence signal from the received signal.
Since the M sequence (synchronizing signal) can be detected from the received signal surely, data can be transmitted at an accurate timing with a described portion of the received signal.